Combine the powdered sugar, milk, and corn syrup in a mixing bowl.

Start slowly, then whisk briskly so the sugar dissolves until the icing becomes smooth and glossy.

Stir until the texture is thick but pourable; if too thick, add a tiny splash of milk, if too thin, add more powdered sugar.

If desired, add food coloring and stir until the tone is even.

Use the icing immediately for piping outlines or flooding cookies.

Let the iced cookies dry on a flat tray at room temperature until firm, usually a few hours.